• A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice

      Smith, I.B.; Lalich, D.E.; Rezza, C.; Horgan, B.H.N.; Whitten, J.L.; Nerozzi, S.; Holt, J.W.; Lunar and Planetary Laboratory, University of Arizona; Department of Geosciences, University of Arizona (John Wiley and Sons Inc, 2021)
      Bright radar reflections observed beneath the south polar layered deposits (SPLD) by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument were interpreted to represent liquid water, but the required amounts of salt and heat to form and maintain liquids in this location are implausible given what is known about Mars. Here, we present another hypothesis that accounts for the bright reflections: hydrated and cold clay-rich deposits at the base of the SPLD create the observed radar response. To support this hypothesis, we present experimental measurements and wave propagation modeling that show that smectites, cooled to 230 K, have real and imaginary parts of the dielectric permittivity large enough to cause the bright reflections, even when mixed with other materials. Further, we find that absorptions attributable to these minerals are present in south polar orbital visible-near infrared reflectance spectra. Because these minerals are present at the south pole and can cause the reflections, we believe this to be a more viable scenario than the liquid water interpretation. © 2021. American Geophysical Union. All Rights Reserved.
    • A Very Young Age for True Polar Wander on Europa From Related Fracturing

      Schenk, Paul; Matsuyama, Isamu; Nimmo, Francis; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2020-09)
      En echelon fissures 100-300 km long on Europa are found to be concentric and external to arcuate troughs previously attributed to true polar wander (TPW) of Europa's ice shell, strengthening the case for TPW. Fissures are composed of parallel faults distributed over 10-to-20-km-wide zones, with deformation focused in a main fissure 1-2 km wide and up to 200 m deep. Fissures crosscut all known terrains, including (apparently) ejecta of bright ray crater Manannan, establishing that fissures and by inference TPW are among the most recent geologic events on Europa. Very late similar to 70 degrees of TPW shell rotation requires that most observed structures on Europa are not in their original configuration with respect to other stress regimes, requiring complete reanalysis of Europa's strain history. If reorientation happened recently, we predict that any crater distribution asymmetries and shell thickness variations measured by Europa Clipper will be offset from expected equilibrium patterns. Plain Language Summary The large icy ocean world of Europa has a very young surface that has been highly deformed. Recent evidence for "polar wander," or reorientation of the floating outer ice shell away from its original orientation, has been confirmed by the recognition that long fissures are part of the polar wander tectonic pattern and arc among the youngest features on the planet. This means that polar wander occurred very recently and that older features are no longer in their original locations and will require a complete reassessment of Europa's tectonic history.
    • A Global Inventory of Ice‐Related Morphological Features on Dwarf Planet Ceres: Implications for the Evolution and Current State of the Cryosphere

      Sizemore, H. G.; Schmidt, B. E.; Buczkowski, D. A.; Sori, M. M.; Castillo‐Rogez, J. C.; Berman, D. C.; Ahrens, C.; Chilton, H. T.; Hughson, K. H. G.; Duarte, K.; et al. (AMER GEOPHYSICAL UNION, 2019-07-02)
      We present a comprehensive global catalog of the geomorphological features with clear or potential relevance to subsurface ice identified during the Dawn spacecraft's primary and first extended missions at Ceres. We define eight broad feature classes and describe analyses supporting their genetic links to subsurface ice. These classes include relaxed craters; central pit craters; large domes; small mounds; lobate landslides and ejecta; pitted materials; depressions and scarps; and fractures, grooves, and channels. Features in all classes are widely distributed on the dwarf planet, consistent with multiple lines of observational evidence that ice is a key component of Ceres' crust. Independent analyses of multiple feature types suggest rheological and compositional layering may be common in the upper similar to 10 km of the crust. Clustering of features indicates that ice concentration is heterogeneous on nearly all length scales, from similar to 1 km to hundreds of kilometers. Impacts are likely the key driver of heterogeneity, causing progressive devolatilization of the low latitude and midlatitude crust on billion-year timescales but also producing localized enhancements in near surface ice content via excavation of deep ice-rich material and possible facilitation of cryomagmatic and cryovolcanic activity. Impacts and landslides may be the dominant mechanism for ice loss on modern Ceres. Our analysis suggests specific locations where future high-resolution imaging can be used to probe (1) current volatile loss rates and (2) the history of putative cryomagmatic and cryovolcanic features. The Cerean cryosphere and its unique morphology promise to be a rich subject of ongoing research for years to come.
    • Investigation of Charon's Craters With Abrupt Terminus Ejecta, Comparisons With Other Icy Bodies, and Formation Implications

      Robbins, Stuart J.; Runyon, Kirby; Singer, Kelsi N.; Bray, Veronica J.; Beyer, Ross A.; Schenk, Paul; McKinnon, William B.; Grundy, William M.; Nimmo, Francis; Moore, Jeffrey M.; et al. (AMER GEOPHYSICAL UNION, 2018-01)
      On the moon and other airless bodies, ballistically emplaced ejecta transitions from a thinning, continuous inner deposit to become discontinuous beyond approximately one crater radius from the crater rim and can further break into discrete rays and secondary craters. In contrast, on Mars, ejecta often form continuous, distinct, and sometimes thick deposits that transition to a low ridge or escarpment that may be circular or lobate. The Martian ejecta type has been variously termed pancake, rampart, lobate, or layered, and in this work we refer to it as abrupt termini ejecta (ATE). Two main formation mechanisms have been proposed, one requiring interaction of the ejecta with the atmosphere and the other mobilization of near-surface volatiles. ATE morphologies are also unambiguously seen on Ganymede, Europa, Dione, and Tethys, but they are not as common as on Mars. We have identified up to 38 craters on Charon that show signs of ATE, including possible distal ramparts and lobate margins. These ejecta show morphologic and morphometric similarities with other moons in the solar system, which are a subset of the properties observed on Mars. From comparison of these ejecta on Charon and other solar system bodies, we find the strongest support for subsurface volatile mobilization and ejecta fluidization as the main formation mechanism for the ATE, at least on airless, icy worlds. This conclusion comes from the bodies on which they are found, an apparent preference for certain terrains, and the observation that craters with ATE can be near to similarly sized craters that only have gradational ejecta.
    • Long-Term and Inter-annual Mass Changes in the Iceland Ice Cap Determined From GRACE Gravity Using Slepian Functions

      von Hippel, Max; Harig, Christopher; Univ Arizona, Dept Math; Univ Arizona, Dept Geosci (Frontiers Media SA, 2019-07-04)
      The Gravity Recovery and Climate Experiment (GRACE) satellites have measured anomalies in the Earth's time-variable gravity field since 2002, allowing for the measurement of the melting of glaciers due to climate change. Many techniques used with GRACE data have difficulty constraining mass change in small regions, such as Iceland, often requiring broad averaging functions in order to capture trends. These techniques also capture data from nearby regions, causing signal leakage. Alternatively, Slepian functions may solve this problem by optimally concentrating data both in the spatial domain (e.g., Iceland) and spectral domain (i.e., the bandwidth of the data). We use synthetic experiments to show that Slepian functions can capture trends over Iceland without meaningful leakage and influence from ice changes in Greenland. We estimate a mass change over Iceland from GRACE data of approximately -9.3 ± 1.0 Gt/yr between March 2002 and November 2016, with an acceleration of 1.1 ± 0.5 Gt/yr2.
    • Martian Ice Revealed by Modeling of Simple Terraced Crater Formation

      Martellato, E.; Bramson, A. M.; Cremonese, G.; Lucchetti, A.; Marzari, F.; Massironi, M.; Re, C.; Byrne, S.; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2020-07-20)
      Arcadia Planitia, a region in the northern midlatitudes of Mars, displays an uncommonly high abundance of simple craters with a concentric morphology, which is indicative of layering beneath the surface. Radar measurements suggest that the near surface layers could be made of excess water ice. In this study, we select two of these impact structures of similar size (D-c similar to 500 m), model their formation through iSALE shock physics code, and investigate the dependence of the final crater morphology on the material model parameters (cohesion and friction coefficient). Our parameter study shows that the intact and damaged cohesions of the nonporous ice play a fundamental role to obtain a good fit between our models and the topographic profiles taken from the digital terrain models in terms of crater diameter, crater wall inclination, and depth and size of the upper terrace. The central pit shape is instead controlled by the damaged friction coefficient of the basaltic crust, but it is mainly affected by projectile density and speed. Our results confirm that two layers of relatively pure water ice, each with different rheology and porosity, can explain the unique double-terraced morphology of impact craters in Arcadia Planitia. The low values of cohesion we find for the ice might point to snowfall as emplacement mechanism in the region. The different thicknesses of the ice layers in the two crater areas seem to suggest variations in ice deposition and/or evolution history across Arcadia Planitia. Plain Language Summary Impact craters are described by a bowl-shaped morphology at smaller sizes. Any departure from such a shape provides insight into subsurface target properties, including changes in density, strength, water content, porosity, and composition. In particular, the presence of steps (or "terraces") along the walls of simple craters provides a straightforward example of complexity within the planetary crusts, and indicates an abrupt transition from upper, weaker layers to deeper, stronger material. Using numerical modeling, we studied two examples of terraced craters in Arcadia Planitia, Mars, to derive information about the rheological properties of the upper Martian crust. This analysis supports radar remote sensing measurements and suggests shallow ice-rich layers in Martian midlatitudes terrains could plausibly cause the terraces observed in these craters. The distribution and properties of water ice are important for understanding Mars' climatic history, as well as the availability of in-situ resources for future human exploration.
    • A Migration Model for the Polar Spiral Troughs of Mars

      Bramson, A. M.; Byrne, S.; Bapst, J.; Smith, I. B.; McClintock, T.; Univ Arizona, Dept Phys; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2019-04-23)
      Mars' iconic polar spiral troughs are 400-1,000-m-deep depressions in the north polarlayered deposits. As the north polarlayered deposits accumulate, troughs migrate approximately poleward, anti-parallel to the local wind patterns. Insolation is suspected to drive ice retreat through sublimation. Sublimation at the trough wall produces a growing sublimation lag that modulates further retreat; however, winds move material off the retreating slope faces, thinning the lag. Discontinuities in stratigraphy seen by radar highlight Trough Migration Paths (TMPs), which provide a record of the troughs' position, formation, and evolution to the present day. We investigate two adjacent troughs presently near 87 degrees N to evaluate the mass balance conditions at those sites. We constrain the contribution of insolation-induced sublimation to the migration in the observed TMPs. We present a phenomenological model that combines our simulations of the sublimation conditions at paleo-trough surfaces with accumulation rates to create synthetic TMPs that are tunable to the observations. Models using nominal values of lag diffusivity, albedo, and atmospheric water vapor abundance and in which the trough walls have been covered in a lag on the order of millimeters thick and formed 2.3Myr ago match the observed trough migration and align with expectations of trough ages. Thicker lags, and/or older troughs, would generate TMPs of constant slope, which does not match the observed paths. We demonstrate the viability of our new theoretical model for predicting conditions that lead to trough migration, allowing us to connect observable TMPs to Martian climate processes. Plain Language Summary Mars' iconic polar spiral troughs are depressions in Mars' northern polar ice cap. The positions of these troughs have migrated poleward over time. Exposure to the Sun's radiation is suspected to drive retreat of the ice through sublimation (ice transitioning directly into vapor without a liquid phase). When ice sublimates, it leaves behind any dust that was within the ice, protecting the ice from further sublimation. Winds, however, have thinned this dust cover, allowing the troughs to continue migrating. Ice layering in subsurface radar data map out the migration paths of the troughs, which provide a record of the troughs' position from their formation to the present day. We investigate ice stability conditions at two adjacent troughs and present a new theoretical model for trough migration. We model sublimation of the trough walls and combine our simulations with previously proposed ice accumulation rates for Mars' north pole to create synthetic trough migration paths. In comparing our models to the observations of trough migration, we find that the trough walls have been covered in millimeters-thick dust over 2.3Myr, consistent with previously hypothesized ages. Our physical modeling approach allows us to connect the observable trough migration paths to Martian climate processes.
    • Preservation of Midlatitude Ice Sheets on Mars

      Bramson, A. M.; Byrne, Shane; Bapst, J.; Univ Arizona, Lunar & Planetary Lab; Lunar and Planetary Laboratory; University of Arizona; Tucson AZ USA; Lunar and Planetary Laboratory; University of Arizona; Tucson AZ USA; Lunar and Planetary Laboratory; University of Arizona; Tucson AZ USA (AMER GEOPHYSICAL UNION, 2017-11)
      Excess ice with a minimum age of tens of millions of years is widespread in Arcadia Planitia on Mars, and a similar deposit has been found in Utopia Planitia. The conditions that led to the formation and preservation of these midlatitude ice sheets hold clues to past climate and subsurface structure on Mars. We simulate the thermal stability and retreat of buried excess ice sheets over 21Myr of Martian orbital solutions and find that the ice sheets can be orders of magnitude older than the obliquity cycles that are typically thought to drive midlatitude ice deposition and sublimation. Retreat of this ice in the last 4Myr could have contributed similar to 6% of the volume of the north polar layered deposits (NPLD) and more than 10% if the NPLD are older than 4Myr. Matching the measured dielectric constants of the Arcadia and Utopia Planitia deposits requires ice porosities of similar to 25-35%. We model geothermally driven vapor migration through porous ice under Martian temperatures and find that Martian firn may be able to maintain porosity for timescales longer than we predict for retreat of the ice.
    • Radar Reflectivity as a Proxy for the Dust Content of Individual Layers in the Martian North Polar Layered Deposits

      Lalich, D. E.; Holt, J. W.; Smith, I. B.; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2019-07-02)
      The stratigraphy of the north polar layered deposits (NPLD) of Mars is believed to contain a climate record of the recent Amazonian period. However, full utilization of this record is difficult without detailed information regarding the physical properties of the constituent layers. Here we present a method for determining the fractional dust content of individual layers using a combination of orbital radar reflectivity measurements and physical modeling. We apply this method to the upper 500 m of the NPLD at 10 study sites and compare the results to a cap-wide radar-mapped surface. Our results show that reflectivity can vary drastically both geographically and with depth, a result we attribute to changing dust content, though the impact of variable layer thickness cannot be totally discounted. These findings imply large-scale regional patterns in ice and dust accumulation do not remain consistent through time. We also find that current models of Mars's dust cycle and polar ice accumulation consistently underpredict the dust content of layers, indicating that our understanding of dust transport, dust sequestration, or dust preservation remains incomplete. Comparisons of study sites on the NPLD also show that some locations contain fewer radar reflectors than others, meaning they may contain a less complete record of the planet's recent paleoclimate, and any future efforts to use the polar layered deposits as a climate proxy, including in situ measurements, should take this into account by choosing study sites wisely.
    • Short-term variations of Icelandic ice cap mass inferred from cGPS coordinate time series

      Compton, Kathleen; Bennett, Richard A.; Hreinsdóttir, Sigrún; van Dam, Tonie; Bordoni, Andrea; Barletta, Valentina; Spada, Giorgio; Univ Arizona, Dept Geosci; Department of Geosciences; University of Arizona; Tucson Arizona USA; Department of Geosciences; University of Arizona; Tucson Arizona USA; et al. (AMER GEOPHYSICAL UNION, 2017-06)
      As the global climate changes, understanding short-term variations in water storage is increasingly important. Continuously operating Global Positioning System (cGPS) stations in Iceland record annual periodic motionthe elastic response to winter accumulation and spring melt seasonswith peak-to-peak vertical amplitudes over 20 mm for those sites in the Central Highlands. Here for the first time for Iceland, we demonstrate the utility of these cGPS-measured displacements for estimating seasonal and shorter-term ice cap mass changes. We calculate unit responses to each of the five largest ice caps in central Iceland at each of the 62 cGPS locations using an elastic half-space model and estimate ice mass variations from the cGPS time series using a simple least squares inversion scheme. We utilize all three components of motion, taking advantage of the seasonal motion recorded in the horizontal. We remove secular velocities and accelerations and explore the impact that seasonal motions due to atmospheric, hydrologic, and nontidal ocean loading have on our inversion results. Our results match available summer and winter mass balance measurements well, and we reproduce the seasonal stake-based observations of loading and melting within the 1 sigma confidence bounds of the inversion. We identify nonperiodic ice mass changes associated with interannual variability in precipitation and other processes such as increased melting due to reduced ice surface albedo or decreased melting due to ice cap insulation in response to tephra deposition following volcanic eruptions, processes that are not resolved with once or twice-yearly stake measurements.
    • Thermophysical Properties of the North Polar Residual Cap using Mars Global Surveyor Thermal Emission Spectrometer

      Bapst, J.; Byrne, S.; Bandfield, J. L.; Hayne, P. O.; Univ Arizona, Lunar & Planetary Lab (AMER GEOPHYSICAL UNION, 2019-05)
      Using derived temperatures from thermal-infrared instruments aboard orbiting spacecraft, we constrain the thermophysical properties, in the upper few meters, of the north polar residual cap of Mars. In line with previous authors we test a homogeneous thermal model (i.e., depth-independent thermal properties), simulating water ice of varying porosity against observed temperatures. We find that high thermal inertia (>1,000Jm(-2)K(-1)s(1/2) or <40% porosity) provides the best fit for most of the residual cap. Additionally, we test the observed data against models with depth-dependent thermal properties. Models tested converge on similar solutions: we find extensive regions of low surface thermal inertia consistent with a porous layer at the surface (>40% porosity) that densifies with depth into a zero-porosity ice layer at shallow depths (<0.5m). We interpret this as evidence of recent water ice accumulation. Our results along the edge of the residual cap imply that denser (<40% porosity) ice is present at the surface and coincides with lower albedo. These results suggest that older ice is undergoing exhumation along much of the residual cap margin. The results support recent water ice accumulation having occurred over specific regions, while ablation dominates in others. Plain Language Summary The polar regions of Mars host kilometer-thick stacks of water ice that have been built up over millions of years. At the north pole today, the top of this ice deposit is interacting with the Martian atmosphere. Whether or not ice at the surface is fluffy (like snow) or dense (like an ice slab) can provide useful information about the polar ice cap and recent climate. Multiple years of surface temperature measurements have been acquired by instruments aboard spacecraft in orbit around Mars. By comparing these values with temperature simulations, we can narrow down the type of ice near the surface. Our results show that the type of ice varies across the polar cap. Some regions appear to be a snow-like surface where the polar cap may be growing. Other regions, most notably along the edge of the polar cap, show denser ice that is likely older. The nature of the ice tells us about the current climate and how these kilometer-thick ice deposits form.